fresh install on a macbook late 2009

first attempt to install xenial xerus xubuntu on a mac late 2009, which was already equipped with a leopard

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2
Architecture: i386
CasperVersion: 1.376
Date: Thu Jun 2 12:54:23 2016
InstallCmdLine: BOOT_IMAGE=(loop)/casper/vmlinuz root=UUID=13DA-9AFA maybe-ubiquity debian-installer/language=fr keyboard-configuration/layoutcode= iso-scan/filename=/xubuntu-16.04-desktop-i386.iso boot=casper file=/cdrom/preseed/xubuntu.seed noprompt quiet splash --
LiveMediaBuild: Xubuntu 16.04 LTS "Xenial Xerus" - Release i386 (20160420.1)
ProcEnviron:
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)